Permit concurrent loads in gpg keychain mutex

What does this MR do?

Wrap MUTEX.synchronize in lib/gitlab/gpg.rb in a permit_concurrent_loads for Rails 5. This fixes new timeouts in the Rails 5 specs: https://gitlab.com/gitlab-org/gitlab-ce/pipelines/26708054/failures

The fix is similar to !20473 (merged) and I think it is now needed because !20524 (merged) is merged and this code is now called more often.

Can be reproduced locally by running: RAILS5=1 bin/rspec spec/features/issues/filtered_search/filter_issues_spec.rb:84 spec/features/projects/files/user_replaces_files_spec.rb:26

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

rails5 upgrade

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conforms to the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

What are the relevant issue numbers?